1. Field of the Invention
This invention is related to a semiconductor laser device and more particularly to a high-power laser device and a fabricating method thereof.
2. Description of the Prior Art
In a conventional high-power laser device of an AlGaAs/GaAs system, the mirror facets at both the ends of the laser device absorb the laser beam and this sometimes results in melting or demolition of the crystal. This phenomenon is called COD (Catastrophic Optical Damage) and it limits the available power level of the laser device. It is known in the prior art that it is effective for increasing the available power level limited by COD to provide a laser device with a very thin active layer in order to lower the power density at the mirror. The power density is lowered because of the weaker confinement of light in the thinner active layer. As the active layer becomes thinner, however, the threshold current increases because of decrease in the gain.
Referring to FIG. 1, there is schematically shown a perspective view of an improved AlGaAs/GaAs laser device with a part cut away, which is described in ELECTRONICS LETTERS, Feb. 13th 1986, Vol. 22, No. 4, pp. 217-218 by T. Murakami et al. An n-GaAs current-blocking layer 5, a p-AlGaAs clad layer 2, a p-AlGaAs active layer 3, an n-AlGaAs clad layer 4 and an n-GaAs contact layer 7 are stacked in this order on a substrate 1 which has a ridge on its top surface from one end to the opposite end. The width of the ridge is narrower in regions 8a near both the end and much wider in an middle region 8b. The end surfaces of the substrate are of (011) and (011) while the side surfaces are of (011) and (011). The current-blocking layer 5 is provided with a slit opened by a narrow groove 10 along the center axis of the ridge. This slit provides a stripe structure for current in the laser device.
In the meantime, it is known that the crystal growth rate in liquid phase epitaxy depends on the surface orientation of the substrate.
Referring to FIG. 2, there is shown a sectional view illustrating an epitaxial crystal growth on a ridged substrate. In this figure, an epitaxial layer 11 of AlGaAs is grown on a substrate 1 of GaAs which has a narrow ridge 8a on its top surface. As seen, the AlGaAs epitaxial layer 11 grows faster on the (111) and (111) side facets of the ridge 8a than on the (100) top facet and thus becomes thicker on those side facets. At this time, since the concentration of As is decreased in the liquid phase just above the narrow top facet of the ridge, the growth rate of the AlGaAs layer 11 is further decreased over the narrow top facet. The growth rate of the epitaxial layer over the ridge depends on the hight h and width w of the ridge as well as on the other conditions, e.g., temperature, oversaturation, etc. Therefore, the thickness d of the epitaxial layer 11 on the ridge can be controlled by selecting the hight h and the width w of the ridge.
Referring to FIG. 3, there is shown a perspective view of the substrate and the current-blocking layer thereon in the laser device of FIG. 1. The same reference characters are used in this figure as in FIG. 1 for the corresponding portions. The ridge is made 20 .mu.m wide in the regions 8a near both the ends and 150 .mu.m wide in the middle region 8b. The active layer 3 of p-AlGaAs grown epitaxially with this substrate has a tapered thickness which is thinner in portions 3a just above the narrower ridge region 8a and thicker in a portion 3b just above the wider ridge region 8b. For example, the active layer portions 3a above the narrower ridge regions 8a can be controlled as thin as 0.04 .mu.m while the active layer portion 3b above the wider ridge region 8b can be made as thick as 0.06 .mu.m.
In the laser device of FIG. 1, the optical power density is lowered in the thinner active layer portions 3a near the mirror facets, while increase of the threshold current can be suppressed with the larger part 3b of the active layer being made thicker. As a result, continuous output of 30 mW can be obtained at a relatively higher temperature of 60.degree. C. with a threshold current of 40-50 mA.
As a matter of fact, however, it is difficult to make a sufficient difference in thickness between the active layer portions 3a and 3b with good reproducibility only by controlling the width of the ridge regions 8a and 8b.
Incidentally, Japanese Patent Laying-Open Gazette No. 60-140774 shows a substrate of a shape similar to that of the present invention. However, the invention of this Gazette is directed to a semiconductor laser with reduced wavefront aberration. In such a semiconductor device, an active layer is formed to preferably have a uniform thickness along the wave guide.